Thin film transistors (TFTs) are widely used as a switching element for a display such as a liquid crystal display device. The representative TFT has a gate electrode, an insulating layer and an organic semiconductor layer on a substrate sequentially and has a source electrode and a drain electrode formed on the organic semiconductor layer with a predetermined distance. The organic semiconductor layer forms a channel region, and an electric current flowing between the source electrode and the drain electrode is controlled by applying a voltage to the gate electrode to allow the on/off action.
Heretofore, the TFT has been made of amorphous silicon or polycrystalline silicon. However, a CVD apparatus which is used for production of the TFT using such silicon is very expensive, and to enlarge a display apparatus or the like using TFTs had a problem of significant increase in the production costs. Further, the process of making the amorphous or polycrystalline silicon into a film is carried out at a very high temperature so that materials usable for a substrate are limited. Thus, there is a problem that a lightweight substrate such as a resin substrate cannot be used.
In order to solve the problems, a TFT using an organic substance (hereinafter often referred to as an “organic TFT”) in place of the amorphous or polycrystalline silicon has been proposed. As a film-forming methods used for production of a TFT with an organic substance, vacuum deposition, a coating method and the like are known. By the use of these film-forming methods, enlarging a device is feasible while suppressing an increase in production cost. Further, the process temperature required at the time of film-forming can be kept to relatively low temperature. By this, the organic TFT has an advantage of small limitation in the selection of materials used for a substrate. Therefore, practical application of the organic TFT is expected, and extensive studies and reports are made.
As materials for a p-type FET (field effect transistor) of an organic semiconductor used for an organic TFT, polymers such as conjugated polymers and thiophene polymers, metal phthalocyanine compounds, fused aromatic hydrocarbons such as pentacene, and the like are used alone or in a mixture with other compounds. As materials for an n-type FET, for example, 1,4,5,8-naphthalenetetracarboxyl dianhydride (NTCDA), 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalenetetracarboxyldiimide (NTCDI) and fluorinated phthalocyanine are known.
On the other hand, as a device using electric conduction in the same manner, an organic electroluminescence (EL) device is known. In the organic EL device, a strong electric field of 105 V/cm or higher is applied to a very thin film typically having a thickness of 100 nm or less in the film thickness direction to compulsorily flow charges. On the other hand, in the case of the organic TFT, it is necessary to flow charges over distance of several μm or more at a high velocity in an electric field of 105 V/cm or lower. Therefore, the organic substance itself used for an organic TFT needs to have a higher conductivity. However, the above-mentioned conventional compounds used for an organic TFT have a small field effect mobility and slow response speed, thus, they have a problem that a high response speed required in a transistor cannot be obtained. Also, the on/off ratios thereof are small.
Here, the “on/off ratio” means a value obtained by dividing an electric current flowing between the source-drain when applying a gate voltage (on) by an electric current flowing between the source-drain when applying no gate voltage (off). The “on” current usually means an electric current value (saturated electric current) at the time when an electric current flowing between the source-drain reaches saturation with a gradual increase of the gate voltage.
As the representative examples of fused aromatic hydrocarbons in the field of electronic materials, anthracene can be given. Non-Patent Document 1 discloses an organic TFT fabricated by using a single crystal of anthracene, and states that this TFT has a field effect mobility of about 0.02 cm2/Vs. However, since anthracene itself is low in solubility and boiling point (sublimation point), it is not suited to a process such as deposition and coating which are normally used in the fabrication of a TFT.
In an organic TFT, a compound which is crystalline but has a high solubility is required. In particular, no organic TFT has heretofore been fabricated using analogues of anthracene such as one having an anthracene ring alone as a ring structure.
For example, a TFT is fabricated using a compound having two or more ring structures like an anthracene oligomer disclosed in Patent Document 1. This TFT is fabricated only by a deposition process since this anthracene oligomer does not have solubility.    Patent Document 1: JP-A-2004-107257    Non-Patent Document 1: Applied Physics Letters, 84, 5383 (2004)
The invention has been made in order to solve the above-mentioned problem, and an object thereof is to provide a compound for an organic thin film transistor which is suited to a coating process. Further, the invention is aimed at providing an organic thin film transistor using it as an organic semiconductor layer.